1. Field of the Invention
The invention relates to a microlithography objective that provides a light path for a light bundle from an object field in an object plane to an image field in an image plane, a projection exposure apparatus with such an projection exposure objective and a usage of such a projection exposure system for processing of chips.
2. Description of the Prior Art
Lithography with wavelengths of <193 nm, particularly EUV lithography with λ =11 nm or λ=13 nm are discussed as possible techniques for imaging of structures of <130 nm, and more preferably of <100 nm. The resolution of a lithographic system is described by the following equation:
  RES  =            k      1        ·          λ      NA      wherein k1 denotes a specific parameter of the lithography process, λ denotes the wavelength of the incident light and NA denotes the numerical aperture of the system on the image side.
For imaging systems in the EUV range, reflective systems with multilayers are used substantially as optical components. Preferably, Mo/Be systems are used as multilayer systems for λ=11 nm and Mo/Si systems are used for λ=13 nm.
The reflectivity of the multilayer systems used currently lies in the range of approximately 70%. Therefore a projection objective for EUV microlithography should have as few optical components as possible to achieve a sufficient light intensity.
In order to achieve a resolution that is as high as possible, on the other hand, it is necessary that the system has an aperture that is as large as possible on the image side.
For lithography systems, it is advantageous if the beam path or so called light path within a projection objective is free of shadows or obscurations. The projection objectives should have no mirrors with transmissive areas, especially openings, since transmissive areas lead to shading. If the objective does not have mirrors with transmissive areas, then the objective has an obscuration-free beam path and the exit pupil of the objective is free of shading and free of obscurations. Furthermore, the aperture diaphragm of such an objective does not need to have a shading device. A disadvantage of systems with an exit pupil shaded, e.g., a so-called Schwarzschild mirror systems, is that structures of specific size can be imaged only with restrictions. The exit pupil is defined as the image of the aperture diaphragm formed by the optical elements arranged in the light path of the objective between the aperture diaphragm and the image plane.
4-Mirror systems for microlithography have become known, for example, from U.S. Pat. No. 5,315,629 or EP 0 480,617 B1. Such systems, however, permit a numerical aperture only of NA=0.1 on the image side with a sufficient field size of at least 1.0 mm scanning slit width. The limit of resolution lies in the range of 70 nm with the use of x-ray light with a wavelength of 10 to 30 nm.
6-Mirror systems for microlithography have been made known from the publications U.S. Pat. No. 5,153,898; EP-A-0 252,734; EP-A-0 947,882; US-A-5,686,728; EP 0 779,528; U.S. Pat. No. 5,815,310; WO 99/57606; and U.S. Pat. No. 6,033,079.
Such 6-mirror systems have a numerical aperture of <0.3 on the image side, which leads to a resolution limit in the range of 30 nm with the use of x-ray light with a wavelength of 10–30 nm.
Another disadvantage of both 4-mirror and 6-mirror systems is the fact that they provide only a few possibilities for correction of imaging errors.
A projection objective for microlithography with eight mirrors has become known from U.S. Pat. No. 5,686,728. This projection objective has a high numerical aperture of NA=0.55 on the image side. Of course, a projection objective as it is known from U.S. Pat. No. 5,686,728 is suitable only for wavelengths longer than 126 nm, since, for example, the angle of incidence of the chief ray of the field point, which lies on the axis of symmetry in the center of the object field, is so large that this 8-mirror system cannot be operated in the EUV wavelength region of 10 to 30 nm. Another disadvantage of the system according to U.S. Pat. No. 5,686,728 is the fact that all eight mirrors are made aspheric and that the angle of the chief ray at the object has a value of 13° with a numerical aperture of 0.11 on the object side.